• theoretical physics
• mechanics
• matter
• energy
• electron
• discrete
• electromagnetism
• continuous
• conceptual tensions
• tube of force

The present dissertation takes into account electromagnetic theories developed in Great Britain in late nineteenth century, after Maxwell published his mature theory. I have confined my research to the years starting from 1881 to early 1890s. In 1881, the second edition of Maxwell’s Treatise appeared, the first chapters revised by Maxwell himself. In 1893 and 1894, J.J. Thomson and J. Larmor published new important theoretical contributions to the interpretation of electromagnetic phenomena. In particular, I have focused on the debate about two basic entities of physics: matter and energy. Those contributions led to important transformations in the concepts of matter and energy and, moreover, led the two concepts closer to each other. The expression “crossing the boundaries”, in the title page, can be interpreted in two ways. First, both matter and energy underwent a conceptual change, wherein matter underwent a sort of dematerialisation and energy underwent a sort of materialisation. Second, some theoretical models broke the borderline between continuous and discrete models, both for matter and for energy.